Electric contact material and method of making the same



. rially oxidizing silver-cadmium alloys.

Patented Feb. 16, 1954 ELECTRIG CONTACT MATERIAL AND METHOD OF MAKING THE SAME Earl I. Larsen; Arnold S. Duty.. and bo nald A. Williams, Indianapolis, Ind., assignors to P. R. Mallory & 00., Inc., Indianapolis, Ind., a cor-'- notation of Delaware No Drawing. Application January 9, 1951 Serial No. 205,232

This invention relates to electric contact materialsand, mor particularly, to electrical make and break contacts and to a method of making such contacts. 5

Electrical make and break contacts composed of silver and cadmium oxide have been in use. for several years and have found wide application on low voltage-high current relays, such as those used in aircraft, as well as in industrial. applications, such as motor starting switches, and the like. Contacts of this type have been formed by powder metallurgical procedures from silver and cadmium oxide powders or preferably by inter- .As div closed in the co-pending application of Duty and Lynch, Serial No. 607,676, filed July 28, 1945, now Patent No. 2,539,298, issued January 23, 1951, when a silver-cadmium alloy is heated in airor in an oxidizing atmosphere at a temperature above 300 C. and below the melting point of 1 Claim. (Cl. 75-'173) not only the good contact properties of silv rsilver and cadmium oxide with the cadmium oxide dispersed throughout th silver matrix in finely divided particles. In general, the silvercadmium oxide contact niaterials produced by internal oxidation are characterized by a density, ultimate strength, ductility and resistance to electrical erosion which are considerably higher than those of silver-cadmium oxide contacts of iden tical composition formed by powder metallurgical procedures.

While silver-cadmium oxide contacts have many' excellent qualities as a result of which they have met with wide public acceptance within a short time, they have the disadvantage of relatively low hardness and are thereby subject to considerable amount of wear and deformation, especially in those applications where contact forces are high and where the contacts close under high impact forces.

It is an object of the present invention to improve electrical contacts of the described character.

It is another object of the invention to pro vide electrical make and break contacts combining the advantages of prior silver-cadmium oxide contacts with the highly desirable characteristic of considerably higher hardness.

The invention also contemplates a method of producing the contact compositions of the invention on a practical and industrial scale at a low cost.

' ness and non-annealing characteristics.

cadmium oxide. but combines such properties with appreciable hardness, thereby overcoming the objectionable feature of low hardness of prior contact materials composed of silver cadmium oxide, in the absence of magnesium oxide.

1' We have further found that the most satisfactory method of producing the contacts of the invention is by internally oxidizing an alloy of silver, cadmium and magnesium. This may be accomplished by heating the alloy at a suitable temperature in an oxidizing atmosphere. It appears that during this treatment oxygen is absorbed into the alloy and will combine with the cadmium and the magnesium to form cadmium oxide and magnesium oxide, respectively but will not combine with the silver. Thus, there sulting product will comprise .a-silver matrix with the cadmium oxide and magnesium oxide par ticles uniformly dispersed throughout the said matrix and will be characterized by high halrhd e non-annealing characteristics of the silver-cadmium oxide-magnesium oxide contacts of the invention make it possible to silver-braze the contacts to a suitable backing and still retain the original high hardness of the material.

The useful range of composition of the silvercadmium oxide-magnesium oxide contacts of the invention is l-25% cadmium oxide, 0.05-5% magnesium oxide, balance substantially all silver. The preferred range of composition is 2 to 15% cadmium oxide, 0.1 to 1.0% magnesium oxide. balance silver.

As a specific example, a contact composed of 89.7% silver, 9.82% cadmium oxide and 0.454% magnesium oxide (by analysis) provided-excellent results as indicated by the results of. the following tests.

The physical properties of this contact composition were the following":

Hardness (st-v1.5 Rockwell F. Ultimate tensil strength 12,000 p. s. i. Elongation 3.1% in 2" length.

Electrical conductivity 74.6% IACS.

In the aircraft relay tests the contacts having 3 the above-mentioned specific composition (89.7% Ag, 9.82% CdO, 0.454% MgO) were subjected to the following test procedure:

V., D. C. 50 operations at 1000 amps 24 50 operations at 1500 amps 24 500 operations at 2000 amps 24 Three sets of contacts were tested. The only failures were that one set stuck after 14 operations at 1500 amps. and welded after 19 operations at 1500 amps. The other two sets went through all the above tests with no failure.

In contrast to this, under similar test conditions, two sets of contacts made from a material composed of 90.0% silver and 10.0% cadmium oxide, but containing no magnesium oxide showed the following:

At 1000 amperes, 1 set of contacts stuck 3 times during the first 35 operations and welded on the 47th operation. Another set stuck on the lstoperaticn, welded on the 47th operation, stuck on the 39th operation and welded on the 52nd operation. Tests were not conducted at higher current due to poor performance at 1000 amps.

The latter (silver and cadmium oxide) material also had a hardness below 57 Rockwell F, actually 74-81 Rockwell I-I. By conversion, these values are below Rockwell B whereas the values obtained on the silver-cadmium oxide-magnesium oxide contacts of the invention convert to an average of about Rockwell 13.

From the foregoing comparative tests it will clearly appear that the contacts of the invention are greatly superior in hardness and in general performance to prior contacts composed of silver and cadmium oxide.

The preferred method of making the contacts of the invention comprises first making an alloy of silver, cadmium and magnesium containing said metals in such proportions that after internal oxidation of the cadmium and magnesium to their respective oxides the resulting product will contain silver, cadmium oxide and magnesium oxide in the desired proportions. The internal oxidation of the alloy is accomplished by heating the alloy in an oxidizing atmosphere, such as oxygen or air, to a temperature higher than 300 C. but lower than the melting point of silver for a suitable period. In general, heatingthe alloy in air at a temperature between 700 and 850 C. for 24-72 hours provides satisfactory results.

In most cases and particularly when forming contacts of small dimensions, it is advantageous to carry out the internal oxidation after the contacts have been brought substantially into their final shape. In other words, a strip is rolled from the starting alloy of silver, cadmium and magnesium and-contacts are blanked from the strip. Finally, the blanked contacts are heated in an oxidizing atmosphere thereby converting them from the silver-cadmium-magnesium alloy into the silver-cadmium oxide-magnesium oxide composition of the invention.

In order that those skilled in the art may have abetter understanding of the invention, the following illustrative example may be given:

Example An alloy of silver-cadmium-magnesium was 4 made by melting the necessary constituents in a graphite crucible using an Ajax induction furnace. The charge consisted of 1265.8 grams silver, grams cadmium, 4.2 grams magnesium. A charcoal cover was used on the melt. The melt was heated to approximately 1230 C. and poured into a carbon mold x 1 x 8%". The resulting ingot was scalped and cold rolled to .100" thick. Reduction per pass during cold rolling amounted to approximately 10%. The bar was annealed after each 35-40% reduction. The .100" thick strip was cold rolled to .040" and suitable contact discs blanked from this strip. The contact discs were then heated for 72 hours at 800 C. in an oxidizing atmosphere so as to internally oxidize the cadmium and magnesium to form cadmium oxide and magnesium oxide respectively.

This treatment resulted in a contact disc having the following properties:

Electrical conductivity 74.8% IACS. Hardness 30 Rockwell B.

Chemical analysis of these contact discs showed the following composition:

89% silver. 8.58% cadmium (9.82% cadmium oxide). 274% magnesium (454% magnesiumoxide).

What is claimed is:

An electrical make-and-break contact formed of a composition consisting of 2-15% of cadmium oxide, 0.l1% of magnesium oxide, the balance being substantially all silver, said composition having a microstructure as is obtained by inter- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,057,604 Zickrick i Oct. 13, 1936 2,145,690 Hensel Jan. 31, 1939 2,161,575 Hensel et al. June 6, 1939 2,207,292 Hensel et al. July 9, 1940 2,396,101 Hensel et al. Mar. 5, 1946 2,539,298 Doty et al Jan. 23, 1951 FOREIGN PATENTS Number Country Date 611,813 Great Britain Nov. 4, 1948 OTHER REFERENCES Rhines et al., Internal Oxidation in Dilute Alloys of Silver Amer. Inst. of Mining and Met. Engrs, vol. 147, 1942, pages 318-330. 

